A wet multi-plate type clutch used in an automatic transmission for an automobile is known from Patent Document 1 below. This clutch includes a clutch drum (clutch outer) 51 spline-joined to a countershaft 12, a clutch spline portion (clutch inner) 23c provided integrally with a gear 23b relatively rotatably fitted to the countershaft 12, a clutch plate 55 and a clutch disk 56 disposed between the clutch drum 51 and the clutch spline portion 23c, and a clutch piston 52 that engages the clutch by making the clutch plate 55 and the clutch disk 56 abut against each other.